The Obama administration is preparing to designate areas in New Mexico and California off-limits to development under its executive authority, according to individuals familiar with the matter, a move that signals a bolder public-lands policy in the president’s second term.

The individuals, who asked not to be identified because a final decision has not been made, said that the White House is poised to act unless Congress moves soon on legislation that will afford similar protections.

One of the two sites, the nearly 500,000-acre Organ Mountains-Desert Peaks region near Las Cruces, N.M., is twice as large as the largest national monument established by President Barack Obama. The other site is about 1,600 acres on California’s central coast known as the Point Arena-Stornetta Public Lands.

Although Congress traditionally designates protection for public lands, presidents have used their authority under the 1906 Antiquities Act to set aside prized areas, known as national monuments.

Obama drew an enthusiastic response from Democrats and conservationists when he said in his State of the Union speech that he would use his authority “to protect more of our pristine federal lands for future generations.”

The monument designations would address a longtime criticism among conservationists that the president has not done enough to safeguard ecologically sensitive and historically important federal lands. They represent one of the most powerful ways he can use his executive powers to achieve policies opposed by Congress.

But such a move is sure to anger some constituencies that rely on public lands for uses such as ranching, mining and motorized-vehicle recreation. They see any new protections as an encroachment on those activities.

Obama started his term by signing a major conservation bill — a bipartisan compromise forged shortly before he took office — but Congress has been largely deadlocked during his tenure. The last Congress was the first since 1942 not to designate a single piece of wilderness as a national park or monument.

The president declared five national monuments last spring, the largest encompassing 242,500 acres. Former Interior Secretary Bruce Babbitt, who served under President Bill Clinton for eight years, suggested that the president put aside one acre of federal land for every acre he leased for oil, gas, coal and mineral extraction. The Center for American Progress, whose chairman, John Podesta, joined the White House in January, issued a report showing that 7.3 million acres of federal land have been leased for oil and gas drilling as of December, while 2.9 million acres have been permanently protected.

Brian O’Donnell, executive director of the Conservation Lands Foundation, said that “there’s no other issue in conservation that fits better” within the president’s new approach of using his executive authority. “Congress has failed to act, the president has the authority, and the communities are ready,” he said.

Rep. Rob Bishop, R-Utah, chairman of the House Natural Resources subcommittee on public lands and environmental regulation, questioned why Obama would invoke the Antiquities Act to protect the Stornetta tract when the House approved adding it to a 1,100-mile marine monument in July.

The unanimous vote on a bill introduced by Rep. Jared Huffman, D-Calif., would add it to the California Coast National Monument. But the Senate has yet to act.

“It would be so much better if you just allowed the Senate to do their work,” Bishop said. “Anything he creates by the Antiquities Act can be undone.”

No monuments have ever been repealed by subsequent presidents, according to former Interior Department solicitor John Leshy, and a 1938 U.S. attorney general’s opinion stated that subsequent presidents lack such power. But in a couple of instances, presidents such as Woodrow Wilson have shrunk their boundaries, and Congress has exercised its power to rescind national monument designations.

The move to designate Organ Mountains-Desert Peaks is more contentious, and more significant. Republicans and Democrats agree that the area has historic, cultural and environmental significance. There are petroglyphs from three American Indian societies in its canyons, as well as desert grasslands and a petrified forest.

Sen. Martin Heinrich, D-N.M., has authored legislation that would create a 498,000-acre national monument, about half of which would be managed as wilderness. Rep. Steve Pearce, R-N.M., however, has proposed a bill that would establish a 54,800-acre monument without any wilderness areas.

While the Senate bill, co-sponsored by Sen. Tom Udall, D-N.M., stipulates that grazing permits would be maintained, local and national ranching groups argue that it would hurt their operations. Some law enforcement officials, such as Dona Ana County Sheriff Todd Garrison, have also said that the move would make it more difficult to monitor illegal activity near the Mexican border.